1. Field of the Invention
The present invention relates to a switching amplifier, and more particularly to a switching amplifier for use as a class-D audio amplifier.
2. Description of the Related Art
A switching amplifier, which is also commonly called a “digital amplifier”, includes a pulse width modulation (PWM) circuit, a driver, a switching output circuit, and a low-pass filter (LPF). The PWM circuit outputs a PWM signal having a pulse width according to the level of the input signal. The driver outputs a driving signal according to the PWM signal. The switching output circuit includes a power MOS-FET connected to a positive power supply, and another power MOS-FET connected to a negative power supply. The driver selectively turns ON one of the power MOS-FETs connected to the positive and negative power supplies in response to the PWM signal so that the switching output circuit outputs a positive or negative power supply voltage.
When a sine-wave input signal is received, if the positive and negative power supply voltages V and −V are constant as shown in FIG. 21, the switching amplifier can produce a regular sine-wave output signal. However, if a power supply voltage is not constant as shown in FIG. 22, the switching amplifier cannot produce a regular sine-wave output signal. Although FIG. 22 shows a case where the negative power supply voltage is constant and only the positive power supply voltage is fluctuating, for the sake of simplicity, it is typically the case that both of the power supply voltages fluctuate.
For example, when a commercial AC power supply is rectified to obtain a DC power supply, the power supply voltage will contain ripples. Since a switching amplifier outputs the power supply voltage as it is, ripples in the power supply voltage will be in the output signal, thus causing so-called “ham noise”.
Moreover, if there is a difference between the absolute value of the positive power supply voltage and that of the negative power supply voltage, a half of that difference will be output (DC offset).
FIG. 23 shows an FFT (Fast Fourier Transform) waveform of an output signal obtained when the input signal is zero where a 50 Hz AC power supply is used. As is apparent from FIG. 23, substantial amounts of ripples having frequency companent of integer multiples of 50 Hz are contained. Although not shown in this figure, a DC offset is also contained.
Various other types of noise may be introduced into a power supply voltage, in addition to those introduced when an AC power supply is rectified to obtain a DC power supply. Then, a switching amplifier will output such noise as it is.
Japanese Patent No. 3394116 discloses a power supply ripple suppressing circuit for a differential amplifier. The differential amplifier is not a switching amplifier but is an ordinary analog amplifier. As shown in FIG. 1 of Japanese Patent No. 3394116, a resistor R12 and an impedance element Z3 are connected in series between the inverted input terminal of the differential amplifier 1 and the ground. An impedance element Z1 is connected between a node T1 (a node between the resistor R12 and the impedance element Z3) and a positive power supply +B, and an impedance element Z2 is connected between the node T1 and a negative power supply −B. Since the differential amplifier is an analog amplifier, impedance value of the impedance element Z1, Z2 are capable of being larger, so that amount of current which flow through the impedance element Z1, Z2 can be smaller. In the case that the differential amplifier is used in a switching amplifier, it is necessary that the impedance values of the impedance element Z1, Z2 are set smaller. In result, since a large amount of power is consumed at the impedance element Z1, Z2, the differential amplifier is not practical as a switching amplifier.